Medical navigation systems, such as image-assisted systems, provide navigational information to a surgeon via a display, for example. The navigational information can be in the form of images and/or text, and can be used by the surgeon to determine the relative location of a particular bone, joint, tumor, etc. of a patient with respect to one or more preoperative images of the patient.
Prior to providing the navigational information, the patient and, more particularly, the area of interest of the patient, is registered. Registration is the process of instructing or teaching the medical navigation system the position of the area of interest in three dimensional space. Once registered, the navigation system correlates the area of interest with the preoperative images of the patient. Moreover, the navigation system tracks the area of interest and provides visual, numerical and/or textual information with respect to the preoperative images on the display. Additionally, the navigation system can display the position of surgical instruments utilized on the patient with respect to the preoperative images in real time.
Navigation systems use a computer connected to one or more tracking sensors or cameras, such that the position of markers fixed to the patient and/or to the instruments can be ascertained, from which the position of the patient and/or instruments can be determined. Such markers, which can include both active emitters and passive reflective markers, can be attached via adapters to a patient and/or to surgical instruments (e.g., a scalpel, forceps, a microscope, a pointer, etc.).
Often, an adapter is used that includes a first fixing element and a second fixing element. The first fixing element, which can be a clamp, is used to attach the adapter to the patient or instrument. The second fixing element is used to attach a reference star. Reference stars are well known in the art and generally include three or four arms extending outwards with active or passive markers attached to the outer ends of the arms. A known navigation system that uses the reference star described above is the image-assisted navigation system VectorVision™, available from BrainLAB AG, and described, for example, in US Patent Publication No. 2003/0225329, which is hereby incorporated by reference.
During surgery, a number of reference stars having different geometries can be arranged within a surgeon's limited working area. The different geometries permit the navigation system to distinguish between each reference star and, therefore, between each instrument or body part attached to the particular reference star. Using a number of reference stars, however, can lead to overlap between reference stars within the cameras' field of vision and/or to individual markers or entire reference stars being obscured from the cameras' field of vision. For example, and with reference to FIGS. 1A and 1B, when the body 2 or an extremity of the patient is moved, the movement can lead to the reference star 4 or a portion thereof exiting the field of vision 6 of the cameras 8. If, in order to re-position the reference star 4 within the field of vision 6, the reference star adapter 10 were detached from the body 2 and then reattached such that the reference star 4 were within the field of vision 6, then the patient would have to be re-registered.
FIG. 2 illustrates a flow diagram 20 of prior art steps performed when the reference star moves out of the cameras' field of vision. Beginning at block 22, a reference star adapter 10 is used to rigidly couple a reference star 4 to a body 2, e.g., an instrument or a region of a patient, and placed in an initial position such that the markers 4a of the reference star 4 are within the field of vision 6 of the cameras 8. At block 24, the body is registered in the navigation system using conventional techniques. Registration of a body in a navigation system generally includes identifying numerous points on the body in three dimensional space to the navigation system. Once sufficient points have been identified, the navigation system correlates the data to preoperative images of the patient.
Once registered, the reference star 4 is tracked by the navigation system, as indicated at block 26. Moreover, since the reference star 4 is rigidly coupled to the body 2, the body also is tracked by the navigation system. Should the reference star 4 become obscured from the field of vision 6 of the cameras 8 as indicated at block 28, then the reference star 4 and the body 2 can no longer be tracked by the navigation system. This can occur, for example, when the body 2 is rotated such that all or part of the markers 4a of the reference star 4 are obscured from the field of vision 6 of the cameras 8. Sometimes, this can be corrected by repositioning the cameras 8 of the navigation system, thereby returning the reference star 4 into their field of vision 6, as indicated at block 30. If this does not correct the problem, then at block 32 the reference star 4 is repositioned by decoupling the reference star adapter 10 from the body 2 and re-coupling it such that the reference star 4 again is within the field of vision 6 of the cameras 8. However, since the reference star adapter 10 is decoupled from the body 2, the navigation system loses the position of the body and, consequently, the body 2 must be re-registered as indicated at block 34. Once re-registered, tracking of the reference star 4 and the body 2 can continue, as indicated at block 36.